Relay used in telegraphy acting as a form of amplifier for the signal, sensitive coils moved the contacts which controlled equipment that could be powered locally. Post Office Standard Relay “B”
This relay replaced earlier forms known as “A” having coils of 100 ohms instead of 200 and being shorter in length. Relay “B” also sometimes having differential coils (two winding on each coil) It can be of the polarised (see item A0239) or non polarised type the polarised version has a magnet and can be neutralised, as in use with the double plate sounder item A0227, provided it has double throw contacts, which means the pole or tongue “T” held between M(mark) and S(space) contacts. The relay shown only has a single throw contact, and is not polarised (no Magnet).

Types of telegraphy relays

“A” Relay earlier version of relay “B”.
“B” Relay can be polarised or none polarised. With differential coils. And single or double throw contacts.
“C” Relay larger magnet and similar to “B” Relay
“D” Relay not polarised and does not have differential coils. (Much cheaper).
“G” Relay later version of Gulstad Relay